This relates generally to image sensors, and more specifically, to methods and circuitry for directly injecting test patterns into individual photodiodes within a pixel array to test the integrity of the components in the pixel array.
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. Conventional image sensors are fabricated on a semiconductor substrate using complementary metal-oxide-semiconductor (CMOS) technology or charge-coupled device (CCD) technology. The image sensors may include an array of image sensor pixels each of which includes a photodiode and other operational circuitry such as transistors formed in the substrate.
Capturing images using a CMOS image sensor often involves using an electronic rolling shutter (ERS) operation to successively reset, integrate, and read out single rows of image pixels on the image sensor. In the traditional ERS operation, row reset and readout are typically performed for a single row at a given time. Row reset refers to an operation which prepares a pixel for light capture by resetting a light generated charge accumulating device (e.g., photodiode) to an uncharged state. Row readout refers to an operation on image pixels that have been exposed to light for a desired duration of time, which involves sampling the pixel columns of a given row and converting a value related to the amount of charge accumulated by the pixel during exposure to a digital signal.
Over the lifetime of an electronic device, image sensors in the electronic device may be prone to failure. Conventional image sensors are sometimes provided with methods for testing the functionality of the image sensor when the electronic device is in stand-by mode. However, failure or error may occur during active operation of the device or may otherwise be undetectable during stand-by mode.
It would therefore be desirable to be able to provide improved image sensors that are capable of verifying the functionality of the imaging system during both stand-by mode and during active operation.